1. Field of the Present Invention
The present invention is directed to a purification technique for treating the reaction mixture comprising a diaryl alkylphosphonate.
2. Description of the Prior Art
Diaryl alkylphosphonates can be prepared by the reaction of an alcohol, such as methanol, with an appropriate triaryl phosphite, for example triphenyl phosphite, in the presence of a rearrangement catalyst. Suitable rearrangement catalysts include strong acids, such as paratoluenesulfonic acid, perfluorooctanesulfonic acid, F.sub.3 CCO.sub.2 H, or alkyl halides, such as methyl iodide. Examples of processes which use such an approach include: German Patent Application No. 2,747,554; M. L. Honig et al., Journal of Organic Chemistry, Vol. 42, 379 (1977); and U.S. Pat. No. 4,152,373 to M. L. Honig et al. The triaryl phosphite that can be used in such a procedure includes aryl radicals containing from about 6 to about 14 carbon atoms, inclusive. Illustrative groups of this type include phenyl, naphthyl, anthracyl, phenanthryl, and the like. The aryl radicals can be either substituted with non-interfering substituents such as alkyl, phenyl, chloro, bromo, alkoxy, aryloxy, alkylthio, arylthio, cyano, nitro and/or hydroxy, or they can be unsubstituted. The alcohol which is used is preferably a lower alkyl (e.g., C.sub.1 to C.sub.4) alcohol with methanol being representative. The weight ratio of the phosphite to the alcohol can range from about 2:1 to about 1:2 parts by weight. However, it is preferable to employ a slight excess of the alcohol over the stoichiometric amount needed. Temperatures used in the reaction can range from about 150.degree. C. to about 300.degree. C. The catalytic amount of rearrangement catalyst can range from about 0.1% to about 10% by weight, with ranges of from about 0.5% to about 5% by weight being generally employed.
It has been found that when the above type of reaction is conducted, the later distillation of the desired product, diaryl alkylphosphonate, from the reaction mixture is greatly complicated by the additional presence, for example, in the reaction residue of unreacted triaryl phosphite starting material and undesired by-product diaryl phosphite. Moisture can cause the triaryl phosphite starting material to act to form the unwanted diaryl phosphite by-product. The diaryl phosphite by-product is believed to be the main cause of unwanted color formation in the reaction residue, smoking of the reaction residue, and the forming of unwanted pyrophoric moieties in the reaction residue when the residue is subjected to distillation to remove and recover the desired product from the reaction mixture. At the start of the distillation procedure, the presence of such unwanted by-products as diaryl phosphite and of unreacted triaryl phosphite is of much less a problem than later on in the distillation procedure. At a later point in the distillation procedure, however, the relative concentration of the diaryl phosphite, for example, and triaryl phosphite in the residue is greatly increased as the desired product is removed from the residue. At some point in the distillation problems can arise due to color formation, smoking, or the formation of pyrophoric substances. The present invention provides a means for the removal, for example, of undesired by-product, e.g. diaryl phosphite by-product and unreacted triaryl phosphite, from the above-mentioned type of reaction residue.